In general, permanent magnet electric motors are broadly classified into two categories, that is, “surface permanent magnet motor” in which a permanent magnet is attached on the outer part of a rotor core, and “interior permanent magnet motor” in which a permanent magnet is embedded in a rotor core. Among these, the “interior permanent magnet motor” is suitable for use as a variable speed drive motor.
In a permanent magnet electric motor, the flux linkage of a permanent magnet is generated constantly and uniformly, as a result, an induced voltage due to the permanent magnet increases in proportion to the rotational speed. Consequently, during a variable speed operation from a low speed to a high speed, a high speed rotation causes a very high induced voltage (counter electromotive voltage) due to the permanent magnet. If the induced voltage due to the permanent magnet is applied to an electronic component of an inverter and increases up to equal to or more than the withstand voltage thereof, the electronic component results in dielectric breakdown. For avoiding such a result, a design is considered, in which the magnetic flux of a permanent magnet is reduced such that an induced voltage due to a permanent magnet is equal to or less than the withstand voltage of an electronic component of an inverter, but this design causes a reduction of output and efficiency in a low speed range of the permanent magnet electric motor.
During a variable speed operation approximating to constant output from a low speed to a high speed, the flux linkage of a permanent magnet is generated uniformly, in a high speed rotation range, the voltage of the motor reaches the upper limit of the power supply voltage and the current required for output is not to flow. As a result, in the high speed rotation range, the output is reduced significantly, and furthermore, a variable speed operation can not be performed over a wide range up to a high speed rotation
Recently, “flux weakening control”, which is described in various documents, has come to be applied to a method for enlarging the variable speed range. The total flux linkage quantity of an armature coil is determined from the magnetic flux due to a d-axis current and the magnetic flux due to a permanent magnet thereof. In the “flux weakening control”, a magnetic flux is generated by a minus d-axis current, and this flux due to the minus d-axis current causes reduction of the total flux linkage quantity. Further, in the “flux weakening control”, a permanent magnet with a high coercivity is adopted such that a working point of a magnetic characteristic (B-H characteristic) may change in the reversible range. Consequently, an NdFeB magnet with a high coercivity is applied to that of a permanent magnet electric motor such that the permanent magnet is irreversibly demagnetized by a demagnetizing field under the “flux weakening control”.
In operations applying the “flux weakening control”, the flux due to the minus d-axis current causes reduction of the flux linkage quantity, as a result, the reduced quantity of flux linkage generates a voltage to spare in relation to the upper limit voltage. An electric current to be a torque component increases, as a result, the output of a high speed range increases. In addition, the rotational speed increases in proportion to the voltage to spare, and this enlarges the range of the variable speed operation.
However, since a minus d-axis current does not contribute to the output, causing the minus d-axis current to flow constantly results in that the copper loss increases and the efficiency is deteriorated. Further, the demagnetizing field due to the minus d-axis current generates a harmonic flux, and increase of voltage due to the harmonic flux or the like makes a limit of voltage reduction due to the “flux weakening control”. For these reasons, even if “flux weakening control” is applied to an “interior permanent magnet motor”, it is impossible to perform a variable speed operation more than three times of a base speed. Further, there is a problem that the harmonic flux as described above causes an increase of iron loss and a substantial reduction of efficiency in a low and middle speed range. In addition, there is a possibility of generating a vibration as a result of electromagnetic force due to the harmonic flux.
When an “interior permanent magnet motor” is applied to a motor for driving a hybrid car, the motor is brought around at the state of only engine drive. In a middle and high speed rotation, an induced voltage of the motor due to a permanent magnet rises, for restraining the induced voltage to be equal to or less than the power supply voltage, a minus d-axis current is caused to flow constantly by the “flux weakening control”. In this state, the motor generates only loss, as a result, an overall operating efficiency deteriorates.
When an “interior permanent magnet motor” is applied to a motor for driving an electric train, since the electric train runs by inertia at a state, similarly as described above, for restraining an induced voltage of the motor due to a permanent magnet to be equal to or less than the power supply voltage, a minus d-axis current is caused to flow constantly by the “flux weakening control”. In this case, the motor generates only loss, as a result, an overall operating efficiency deteriorates.
As a technique for solving these problems, in Patent Document 1 and Patent Document 2, a technique is described, which arranges permanent magnets with a low coercivity such as to change magnetic flux density irreversibly due to a magnetic field generated by a stator coil, and permanent magnets with a high coercivity that is equal to or more than twice as much as that of the permanent magnets with a low coercivity, and adjusts the total flux linkage quantity by magnetizing the permanent magnets with a low coercivity by magnetic fields due to current such that the total flux linkage due to the permanent magnets with a low coercivity and the permanent magnets with a high coercivity is reduced in a high speed rotation range in which the voltage is equal to or more than the maximum voltage of the power supply voltage.
A permanent magnet electric motor of Patent Document 1, has a rotor 1 that is configured as shown in FIG. 9. Here, the rotor 1 is configured by a rotor core 2, eight permanent magnets with a low coercivity 3 and eight permanent magnets with a high coercivity 4. The rotor core 2 is configured by layering silicon steel plates, the low coercivity permanent magnets 3 are Al—Ni—Co magnets or FeCrCo magnets, and the high coercivity permanent magnets 4 are NdFeB magnets.
The low coercivity permanent magnets 3 are embedded in the rotor core 2, and first type of cavities 5 each are provided at the opposite ends of the low coercivity permanent magnets 3. The low coercivity permanent magnets 3 each are arranged along with the radial direction of the rotor that matches the q-axis as a center axis between magnetic poles, and magnetized in the orthogonal direction to the radial direction. The high coercivity permanent magnets 4 are embedded in the rotor core 2, and second type of cavities 6 each are provided at the opposite ends of the high coercivity permanent magnets 4. The high coercivity permanent magnets 4 each are arranged approximately along with the round of the rotor 1 such that the high coercivity permanent magnets 4 each are put between two low coercivity permanent magnets 3 on the inner side of the rotor 1. The high coercivity permanent magnets 4 each are magnetized in the orthogonal direction to the round of the rotor 1.
The magnetic pole parts 7 each are formed such that they are surrounded by two low coercivity permanent magnets 3 and one high coercivity permanent magnet 4. The center axis of the magnetic poles 7 is d-axis, and the center axis between each two magnetic poles 7 is the q-axis. In the permanent magnet electric motor of Patent Document 1 which adopts this rotor 1, pulse-like currents are flowed through stator coils in an extremely short conductive time (about 100 μs to 1 ms) to form magnetic fields, and thereby the magnetic fields act the low coercivity permanent magnets 3. When a magnetized magnetic field is 250 kA/m, ideally, a sufficient magnetized magnetic field act the low coercivity permanent magnets 3, and the high coercivity permanent magnets 4 are not irreversibly demagnetized due to the magnetization.
As a result of this, with the permanent magnet electric motor of Patent Document 1, the flux linkage of the low coercivity permanent magnets 3 changes from the maximum value to zero, and magnetization can be made in the two directions of forward and reverse. In other words, if the flux linkage of the high coercivity permanent magnets 4 is directed in a forward direction, the flux linkage of the low coercivity permanent magnets 3 can be adjusted in a range of from the maximum value of the forward direction to zero, and furthermore, over a wide range up to the maximum value of the reverse direction. Consequently, in the rotor 1, the low coercivity permanent magnets 3 are magnetized by a d-axis current, as a result, the total flux linkage quantity can be adjusted over a wide range, which is obtained by summing up that of the low coercivity permanent magnets 3 and that of the high coercivity permanent magnets 4.
For example, in a low speed range, when the low coercivity permanent magnets 3 are magnetized by a d-axis current such that the flux linkage of the low coercivity permanent magnets 3 shows the maximum value in the same direction (in a initial state) of the flux linkage of the high coercivity permanent magnets 4, the torque due to the permanent magnets reaches the maximum value, and this allows the torque and output of the motor to be the maximum. In a middle and high speed range, when the magnetic flux is reduced and the total flux linkage quantity is reduced, the voltage of the motor is reduced, and this generates a voltage to spare in relation to the upper limit of the power supply voltage, and allows the rotation speed (frequency) to be higher.
Patent Document 1: Japanese Patent Application Laid-open No. 2006-280195 Patent Document 2: Japanese Patent Application Laid-open No. 2008-48514
The permanent magnet electric motor of Patent Document 1 which is configured as described above, has an excellent characteristics that the flux linkage quantity of the low coercivity permanent magnets 3 changes over a wide range of from the maximum value to zero by a d-axis current of the rotor 1, and that and magnetization can be made in the two directions of forward and reverse. On the other hand, a large magnetization current is required when magnetizing the low coercivity permanent magnets 3, this allows an inverter for driving the motor to be enlarged.
In particular, in view of characteristics of permanent magnets, a large magnetization current is required when magnetization compared with when demagnetization. However, since the permanent magnet electric motor of Patent Document 1 has a configuration in which two types of magnets are arranged magnetically in parallel, by the influence of the flux linkage of the high coercivity permanent magnets 4, a large magnetic field is required for magnetization of the low coercivity permanent magnets 3.
FIGS. 10(A) to 10(D) are schematic diagrams illustrating this. In the permanent magnet electric motor of Patent Document 1, as shown in FIG. 10(A), two low coercivity permanent magnets 3 and one high coercivity permanent magnet 4 are arranged in a U-shape such that d-axis is center of the U-shape. In a normal state of the motor, the flux of the permanent magnets 3 and 4 is directed in the direction of the center magnetic pole part 7. In this state, when a d-axis current is flowed in pulse-like to generate a magnetic field for demagnetization, the flux of the magnetic field is generated such that the flux goes through each of the permanent magnets 3 and 4 from the outer side of the rotor 1, as shown in FIG. 10(B), resulting in that the low coercivity permanent magnets 3 are demagnetized. In this case, since the high coercivity permanent magnet 4 has a high coercivity, the magnet 4 is not demagnetized.
During the demagnetization process, as shown in FIG. 10(C), the flux of the high coercivity permanent magnet 4 is flowed in the d-axis direction and in the direction of going from the inner side to outer side of the low coercivity permanent magnets 3, that is, in the reverse direction of an initial direction of the flux of the low coercivity permanent magnets 3, thereby the high coercivity permanent magnet 4 assists the demagnetization due to a magnetic field generated by a d-axis current. Consequently, the demagnetization can be made up to the reverse of the polarity of the low coercivity permanent magnets 3.
On the other hand, when a magnetization process, a d-axis current is applied in pulse-like again, as shown in FIG. 10(D), a magnetic field is generated in the reverse direction to that of FIG. 10(B), the flux in the reverse direction which configures the magnetic field, restores the flux linkage of the demagnetized low coercivity permanent magnets 3 to the state of a normal operation. However, essentially, a large energy is required for the magnetization compared with the de magnetization, in addition to this, as shown in FIG. 10(D), the flux of the high coercivity permanent magnet 4 is applied to the low coercivity permanent magnets 3 in the direction of demagnetizing the low coercivity permanent magnets 3, as a result, a large magnetization currant is required, which can generate a large magnetic field to overcome this.
As described above, in the permanent magnet electric motor of Patent Document 1, since the two types of magnets are arranged magnetically in parallel, there are merits such as a large magnetization of the low coercivity permanent magnets 3 and increase of the variation of the magnetic force such as from zero to 100%, but there is a demerit in which a large magnetization currant is required for a magnetization process.
This invention is proposed to solve the problems described above, and has as an object the provision of a permanent magnet electric motor which can reduce a magnetization current when magnetizing low coercivity permanent magnets, thereby which, without enlarging an inverter, can perform a variable speed operation over a wide range of from a low speed to a high speed, as a result, which can contribute to a high torque in a low speed rotation range, a high power output in a middle and high speed rotation range, and an improvement of efficiency.